1. Field of the Invention
The present invention relates to a new and improved method and apparatus for separating oversize and on size particles in a mass of particulate material which is passed through a screening apparatus. The method and screening apparatus of the present invention is particularly useful in the oil well drilling industry for removing solid particles, particularly finer sands, from oil well drilling fluids.
2. Description of the Prior Art
In accordance with the prior art, most vibrating screens are mounted with the screen cloth at a slope angle of from 15.degree. to 30.degree. above the horizontal. The material is passed over the upper surface of the screen from top to bottom usually under the influence of gravity alone and the flow is relatively slow because of the relatively shallow angle. The particles that are much smaller than the openings in the screen cloth easily pass through the screen mesh with little difficulty and similarly, the particles that are substantially larger than the screen mesh openings are positively rejected and do not pass through the mesh. These oversize particles move readily along and are eventually taken off the lower end of the screen as oversize. The main problem with prior art screening devices of the character described occurs with the particles of a critical particle size range wherein the particles are just slightly greater in size than the openings in the screen cloth. These critical size particles tend to fall into the open mesh, but because they cannot pass through the openings they become wedged solidly in the mesh causing the screen cloth to become blinded or plugged up. When this occurs, the screening operation is highly inefficient and as more material is passed across the screen, repeated collisions causes the particles to become wedged even more firmly. Cleaning of the screen cloth is then required before operations can be efficiently maintained.
In accordance with the present invention, however, the problem of blinding or wedging of critical size particles in the screen cloth openings is greatly reduced or entirely eliminated and the screening action is greatly speeded up.
In the oil well drilling industry, each gallon of drilling fluid that is pumped from the bottom of a hole is run over "shale-shaker" or coarse mesh screen to remove the large chips, cuttings, shale and coarse sand. The material passing through the "shale-shaker" then runs into a reservoir where the finer materials settle to the bottom and are removed with liquid vacuum cleaners. In accordance with the present invention such vacuum cleaners and the energy expanded through their use are unnecessary since the liquid material containing solid particles from the "shale-shaker" can be conveyed onto the screening apparatus disclosed herein to remove the finer sands commonly removed by a liquid vacuum cleaner. The apparatus of the present invention is self cleaning and does not use energy, as would the liquid vacuum cleaner method.